1. Field of the Invention
The present invention relates generally to an optical moisture sensor for mounting upon the interior surface of a windshield, and more particularly, to a compact optical moisture sensor having a optical emitters, detectors, and optical components mounted on a planar circuit board which is positioned parallel to the interior surface. Collimator lenses and a prismatic coupler are used to reflect and refract light beams as the light beams travel from the emitters to the outer surface of the windshield and back to the detectors.
2. Summary of Related Art
Motor vehicles have long been equipped with motor-driven windshield wipers for cleaning moisture from the external surface of the windshield, at least within the driver's field of vision, and generally over a larger area so as to enhance vision through the windshield. In most vehicles today, the windshield wiper system includes multi-position or variable speed switches which allow the driver to select a wide, if not an infinitely variable, range of speeds to suit conditions. Wiper controls are manually operated and typically include a delay feature whereby the wipers operate intermittently at selected time delay intervals.
Wiper control systems have recently been developed which include a moisture sensor mounted on the windshield to automatically activate the motor when moisture is deposited upon the surface of the windshield or other vehicle window upon which a wiper may be employed, such as the rear window. By sensing rain or other moisture on the glass surface, the wipers can be controlled accordingly. Such wiper control systems free the driver from the inconvenience of frequently adjusting the wiper speed as the driving conditions change. Wiper control systems with optical moisture sensors have been incorporated into the production of several models of passenger cars. In order to increase the commercial use and consumer acceptance of the wiper control systems, there is a need for a more compact and less expensive optical moisture sensor.
Wiper control systems have employed a number of different technologies to sense the moisture conditions encountered by a vehicle, including conductive, capacitive, piezoelectric, and optical sensors. Optical sensors operate upon the principle that a light beam being diffused or deflected from its normal path by the presence of moisture on the exterior surface of the windshield. The systems which employ optical sensors have the singular advantage that the means of sensing (i.e. disturbances in an optical path) is directly related to the phenomena observed by the driver (i.e., disturbances in the optical path that affords the driver vision). Thus, optical systems generally have an advantage over other sensor technologies in that they are closely related to the problem corrected by the wipers.
McCumber et al. (U.S. Pat. No. 4,620,141) disclose an automatic control circuit for triggering a sweep of the wiper blades in response to the presence of water droplets on the exterior surface of a windshield. The rain sensor devices for controlling the windshield wipers of a vehicle as disclosed by McCumber et al. and Teder (U.S. Pat. Nos. 5,059,877 and 5,239,244) include a box-like housing mounted upon the interior surface of the windshield. The presence of moisture on the surface of the windshield affects the reflection of light at the air-glass interface, and this change in reflected light is electronically processed and utilized as the signal for activating the windshield wipers. The sensor housing in an optical moisture sensor should securely engage the windshield and be optically coupled to the windshield so as to effectively eliminate the interface between the light emitters-detectors and glass surface from an optical standpoint. U.S. Pat. No. 5,262,640 to Purvis et al. describes an intermediate adhesive interlayer for affixing the sensor housing to the windshield. The sensor housing is affixed directly to the surface of the windshield or other vehicle window by means of an intermediate interlayer disposed between the sensor housing and the interior surface of the windshield.
In optical moisture sensors, light from an emitter is directed by a guide means into the windshield at an angle of approximately forty-five degrees with respect to the windshield. The light is then reflected by the outer surface of the windshield at approximately a forty-five degree angle and is directed by a guide means into a detector. Water on the outside surface of the windshield effects the overall transmittance of the optical path between emitter and detector.
When the angle of entry of the light beam into the windshield is greater than fifty degrees, a loss of signal frequently occurs. When the angle of entry is less than forty degrees, a loss of sensitivity occurs and the sensor is not able to properly detect moisture on the windshield. Consequently, it is essential that the angle of entry of the light beam from the emitter enter the windshield at approximately forty-five degrees.
The desired forty-five degree angle can be achieved by mounting the optoelectronic devices (emitters and detectors) at forty-five degree angles or by deflecting the light as it travels between the devices and the glass windshield. The sensors in which the emitters and detectors are mounted at forty-five degree angles to the windshield have required bulky, box-like enclosures. Light may be deflected only by reflection, refraction or diffraction. Reflecting mirrors are amenable to deflections of sixty degrees or greater. A mirror designed to implement a shallower deflection must be quite large to accommodate a wide splay of rays. Diffractive lenses are not very efficient and can be quite expensive. A refractive service can efficiently deflect a beam approximately twenty degrees or less. The preferred forty-five degree angle for optical moisture sensors is generally too small for a reflective system and too large for a refractive system. Consequently, most of the optical sensors have used optical devices deployed at a suitable angle rather than devices for deflecting the light at the desired angle.
The references cited above have optical devices deployed at forty-five degrees, which requires a box like enclosure. Additional examples of optical sensor mounting configurations to achieve the forty-five degree angle between the optical axis of the emitter and the glass windshield are disclosed in Noack (U.S. Pat. No. 4,355,271), Bendicks (U.S. Pat. No. 5,323,637) and Larson (U.S. Pat. No. 4,859,867).
Stanton (U.S. Pat. No. 5,414,257) discloses optical sensor optoelectronic devices mounted on a circuit board at an appropriate angle to change or deflect the optical axis. Stanton teaches devices cast from flexible epoxy resin and the bending of the leads to the desired angle. The problem with electronic devices with bent leads is that most automated component insertion equipment cannot insert components with bent leads. In addition to increased costs to assemble the circuit boards, the bent lead devices are less reliable from a performance standpoint.
The mounting of optoelectronic devices on circuit boards is also disclosed in Schierbeek (U.S. Pat. No. 4,956,591) and in Wiegleb et al. (DE3806881). The optoelectronic devices are mounted on small circuit boards which are aligned perpendicular to the windshield. Reflective surfaces, each bending the light ninety degrees in a rotational fashion, deflect the optical axis to the required angle within the windshield. Although the mounting configurations in these references do not require lead forming, the use of such small circuit boards creates other problems. The small circuit boards used to mount the optoelectronic devices cannot accommodate the signal processing circuitry, which must be located on a separate circuit board. The use of multiple circuit boards and the orientation of the circuit boards in the housing of the sensor increases the size and cost of the sensor. The required mounting angles for the optoelectronics in a sensor could also be obtained by the use of flexible circuit boards, but such material is more expensive and less reliable than standard circuit boards.
Optoelectronic devices are customarily mounted and aligned on a printed circuit board, which also accommodates signal processing. Conventional optoelectronic devices, including the new surface-mount technology devices (SMT's), are generally designed so that their optical axis is normal to the circuit board on which they are mounted. The use of a single circuit board mounted coplanar with the surface of the windshield could result in a low cost and compact sensor enclosure. However, such design presents significant problems in achieving the desired forty-five degree configuration because the optical axis is perpendicular to the circuit board.
One configuration which both reduces the cost and reduces the size of the optical sensor is to use a single detector to simultaneously detect two or more emitters, as disclosed in Noack. Such a configuration provides the desired area of detection with a fewer number of detectors. However, the light paths are widely splayed, which requires a larger detector or additional optical elements for concentrating the light.
Another area of concern in the manufacture of optical moisture sensors is the mounting of the sensor to the windshield. Vehicle manufactures desire a sensor which is already installed at the windshield manufacturer, or a sensor that is very easy to install on the vehicle production line. The windshield manufacturer ships windshield nested together such that there is very little spacing for mounting a sensor.
Schofield (U.S. Pat. No. 4,930,742) discloses the use of a bracket, such as a rear view mirror bracket, for mounting the optical moisture sensor. This approach necessitates additional support structure or the addition of silicone pieces to optically couple the moisture sensor to the windshield. A bracket mounting systems results in additional parts and increased costs.
Bendix (U.S. Pat. No. 5,278,425) and Stanton teach that a lens may be permanently affixed to the windshield such that a sensor housing may be detachably mounted on the lens. The lens may impart focal power to the beam, as in shown Bendix. Alternatively, the lens may couple the beams to the windshield through planar surfaces normal to the beam direction, as disclosed in Stanton. However, both Bendix and Stanton require a lens that is approximately as thick as the windshield. When stacking the windshields for transportation from the glass manufacturer to the vehicle assembly line, the additional space necessitated for the lens adds additional handling costs to the cost of the windshield.
Watanabe (U.S. Pat. No. 4,701,613) discloses an integral coupler lens having a series of V-grooves forming a segmented prism with planar surfaces normal to the direction of the beams. Segmented lenses have a greater potential for parasitically admitting ambient light, which reduces optical efficiency and degrades the signal from the emitter. The resulting beam travels at a forty-five degree angle with respect to the windshield, and thus is not amenable to coplanar approaches.